<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        input {
            color: #999;
        }
    </style>
</head>

<body>
    <input type="text" value="手机" placeholder="手机">
    <script>
        var input = document.querySelector('input');
        input.onfocus = function() {
            if(input.value === '手机') {
                input.value = '';
            }
            input.style.color = "#333";
        }
        input.onblur = function() {
            if(input.value === '') {
                input.value = '手机';
            }
            input.style.color = "#999";
        }
    </script>
</body>

</html>